Estradiol priming of hypophysectomized rats enhances the ability of FSH over FSH alone to increase its own receptor. However, estradiol alone does not alter levels of FSH receptor. We are attempting to answer the question of whether estradiol stimulation of the ovary is necessary before FSH can autoregulate the FSH receptor using immature hypophysectomized rats. Although estradiol binds to cytosol and nuclei of granulosa cells or ovarian homogenates, it has not been determined whether the binding kinetics of estradiol to nuclear or cytosolic components of granulosa cells are similar to those previously reported for estradiol receptors in other tissues. It is well known that many blood binders have reasonably high affinities 10 to the minus 8th power, 10 to the minus 9th power M for estrogen. In our study Scatchard analysis indicates that a single class of high affinity (10 to the minus 10th power M) binding sites specific for estradiol exists in granulosa cell cytosol preparations. However, the nucleus seems to contain multiple classes of binding sites, at least one is high affinity (10 to the minus 10th power M) and specific for estradiol. The high affinity site in the nucleus and cytosol is labile at 37 degrees, whereas the lower affinity site(s) is unaffected by heating. Testosterone and progesterone do not compete with labeled estradiol for the high affinity cytosol or nuclear binding site. These data suggest that an estradiol binding protein is present in granulosa cells. This protein is specific for estradiol, has affinities similar to those reported for uterine estradiol receptors and is heat labile. Currently, we are further characterizing this binding factor and attempting to determine if exogenous FSH alone increases the quantity of this binding factor in cytosol or nuclear preparations before, coincident with or after increases in its own receptor.